For several decades, seismic prospecting for petroleum has involved the creation of acoustic disturbances above, upon, or just below the surface of the earth, using explosives, air guns, or large mechanical vibrators. Resulting acoustic waves propagate downwardly in the earth, and are partially reflected back toward the surface when acoustic impedance changes within the earth are encountered. A change from one rock type to another, for example, may be accompanied by an acoustic impedance change, so that the reflectivity of a particular layer depends on the velocity and density content between that layer and the layer which overlies it, say according to the formula EQU C Reflect (normal)=AR/Ai=[(V.sub.2 d.sub.2 -V.sub.1 d.sub.1)/(V.sub.2 d.sub.2 +V.sub.1 d.sub.1)]
where AR is the amplitude from the reflected signal and Ai is the amplitude of the incident signal; V.sub.1 is the velocity of the wave in the overlying medium 1; V.sub.2 is the velocity in the medium layer below the contact line; d.sub.1 is the density of the overlying medium 1; and d.sub.2 is the density of the underlying medium.
In early years, signal traces of the reflected acoustic waves were recorded immediately in the field as visible, side-by-side, dark, wiggly lines on white paper ("seismograms"). At present, the initial reproductions--in a digital format--are on magnetic tape, and finally are reduced to visible side-by-side traces on paper or film in large central computing facilities.
At such centers, sophisticated processing makes possible the distinction of signals from noise in cases that would have seemed hopeless in the early days of seismic prospecting. Until 1965, almost all seismic surveys conducted used an automatic gain control which continuously adjusted the gain of amplifiers in the field to account for decreasing amounts of energy from late reflection arrivals. As a result, reflection coefficients could not be accurately determined. However, with the advent of the expander circuit and binary gain amplifiers, gain of the amplifiers can now be controlled and amplitudes recorded precisely; this makes it possible to conserve not only the special characteristics of the reflections, but also their absolute amplitudes.
Today, more powerful computers with array processors and economical floating point capabilities also now enable modern geophysicists to maintain control of the amplitude of all recorded signals. The "floating point" capability is especially effective in expanding computer work size by a large factor and in eliminating the need for computer automatic gain control.
In summary, as a result of the above advances, reflections from many thousands of feet below the earth's surface can now be confidently detected and followed through sometimes hundreds of side-by-side traces, the shortening or lengthening of their corresponding times of arrival being indicative of the shallowing or deepening of actual sedimentary strata of interest. Still, as a general rule, all that can be hoped for the seismic reflection method is to detect structural interfaces and their deviations from horizontality, so as to define subsurface patterns associated with oil or gas.
Apropos of the above has been use of ultra-high amplitude anomalies in seismic traces to infer the presence of natural gas in situ. Seismic interpreters have used so-called "bright-spot" analysis to indicate several large gas reservoirs in the world, especially in the Gulf Coast of the United States. Such analysis is now rather common in the oil industry, but it is not without its critics. Not only cannot the persistence of such increased amplitude anomalies be taken as confirmation of the lateral extent of the gas reservoir, but also the anomaly itself (in some cases) may not represent reflections of a discontinuity of a gas-bearing medium and its over- or underlying associated rock strata. E.g., experience has shown that in certain situations, similar phenomena occur which can confuse the interpreter. E.g., if the shape of the horizon is such that it focuses the energy back to the surface, it may increase the amplitude of one or more of the records akin to reflections from gas-saturated strata. Lithology of the horizon--singly and in combination--can also have a similar effect, producing high-amplitude reflections in the absence of gas within the pore space of the stratum of interest. Examples of the latter: conglomeratic zones, hard streaks of silt or lime and lignite beds.
The present invention improves the ability of the seismologist to correctly differentiate the lithology associated with high-intensity anomalies of multiple-point-coverage seismic traces of gas-bearing strata from that of similarly patterned reflections of other types of stratigraphic configurations containing no gas accumulations.